The present invention is directed to an improved electrochemical oxidation process for forming quinones and aromatic aldehydes or ketones from corresponding aromatic and alkyl aromatic compounds in good yields and high selectivity. More specifically, the invention described and claimed herein requires the use of a strong aqueous trifluoromethanesulfonic acid solution having high concentrations of ceric trifluoromethanesulfonate dissolved therein.
The quinones and aromatic aldehydes or ketones obtainable by the present process have a wide variety of known utility. For example, the quinones, such as naphthoquinone, are known additives in the paper making industry. The aldehydes, such as benzaldehyde, tolualdehyde and the like, and ketones such as p-methylacetophenone are known intermediates used in forming fragrance components useful in perfumes and colognes. Certain aldehydes and ketones have been used in forming pharmaceuticals.
The products achieved by the present invention have been previously formed by a variety of processes which may be generally classified as chemical or electrochemical. For example, aromatic aldehydes have been chemically formed by air oxidation conducted in an oxygen enriched environment at high temperatures and pressure in the presence of a transition metal catalyst or by using known chemical oxidizing agents which are not regenerable. Oxidation has also been achieved by direct electrochemical oxidation of aromatic compounds in the presence of dilute acid electrolytic solutions as described in U.S. Pat. Nos. 4,298,438 and 4,354,904 and by indirect electrochemical oxidation in which the oxidant is electrolytically generated and, in turn, used to oxidize the aromatic compound.
Compounds which are known to be capable of acting as an indirect oxidant include transition metal salts, particularly the metals of cobalt, chromium, manganese, iron, lead, silver and cerium. Because regeneration of the spent metal to its higher oxidation state is not always highly effective and/or other insoluble salts, such as oxides, etc., are formed, those skilled in this art tend to use the salts of chromium, manganese, cobalt, iron or lead as these salts are less expensive and replacement of spent materials do not greatly detract from the economics of the process. However, each of these metal ion oxidants have certain properties which cause them to make the oxidation process ineffective. For example, chromium ions give poor selectivity towards the desired products, cerium and manganese salts are believed to have low solubility of the oxidized and/or reduced ions in acidic solutions, the higher oxidation states of silver, cobalt and lead ions are not very stable and, in the case of iron, is not very reactive. Indirect electrochemical oxidation has been further complicated by the properties of the anion specie present. For example, certain anions (e.g., chloride, nitrate, perchlorate) are highly reactive with the organic substrate producing by-products or conditions which preclude their use on a commercial scale. Other less reactive anions (e.g., sulfate, acetate, fluoride, boron fluoride, silicon fluoride) generally form salts of low solubility, inhibit the rate of reaction of the oxidant with the organic substrate and/or inhibit the ability of the spent oxidant to be regenerated. In addition, certain organic acid salts (e.g., benzenesulfonate) have been found to be insufficiently stable to be useful in an indirect oxidation process.
Cerium and its ceric ion is a well known oxidizing agent in organic chemistry. It has the potential of presenting an excellent one electron oxidant but has not been previously used extensively or on an industrial scale because of the inability of both the ceric and cerous ions to be maintained in solution at high concentrations and under high acidity causing its use to be limited to slurries or very low concentrations. In addition, ceric oxidant has been associated with poor reactivity and selectivity. The cerium salts are prohibitively expensive and must, therefore, be capable of being stable, react with the organic substrate cleanly and be easily regenerated to its higher valence state. This requires the ceric salt to exhibit a high degree of stability and solubility in the electrolyte solution and be capable of achieving good reaction rates. In addition, the cerous ion must also be highly soluble to be capable of being regenerated to the ceric ion under conditions of high current efficiency at the anodic portion of the electrochemical cell. However, conditions (i.e. high acidity) preferred for best utilization of the ceric ion have previously been believed as being counterproductive to achieving proper conditions for cerous salt utilization. Therefore, it has heretofore been believed necessary to use the cerium salt at very low concentrations and under a very narrow set of conditions including those which could not demonstrate the potential necessary to provide an effective industrially suitable process.
Canadian Pat. No. 1,132,996 to Oehr describes a process for oxidizing naphthalene to naphthaquinone using ceric sulfate in dilute sulfuric acid. Both cerous sulfate and ceric sulfate are known to have low solubility in dilute acid [Solubilities of Inorganic and Organic Compounds, Vol. 3, Part I, Ed. by H. L. Silcock (1974)] and the solubility decreases with increasing acid concentration. The solubility limitations lead to the use of inefficient slurry conditions or to the need for large volumes of solution to oxidize small quantities of the organic compound. Similar problems are encountered with other salts of low solubility.
European Patent Application No. 0075828 of Mayeda et al describes a process for oxidizing fused ring compounds to their respective quinones using ceric nitrate in dilute nitric acid. Although solubility does not present a problem, the nitrate anion is known to react with the organic reactant forming nitrogen containing by-products which are difficult to handle and remove. Cerium salt solutions containing perchlorate anions have also been disclosed as a useful oxidant [Prospects for the Indirect Electrolytic Oxidation of Organics, by N. Ibl et al., AIChE Symposium Series, Electroorganic Synthesis Technology, Pg. 45, (1979)] but it is well known that the perchlorate reacts explosively with organic materials and, therefore, is unsuitable for commercial scale processes.
M. Marrocco et al [J. Org. Chem., Vol. 48, No. 9, Pg. 1487 (1983)] conducted a study of the oxidation of an organic substrate by various cerium salts in different acid electrolytes. Each of the cerium salt systems contained cerium ions maintained at very low concentrations. Even at the low concentrations, many of the systems were slurries and the conversion and selectivity were low.
It must be understood that although cerous/ceric ions have been known and used in oxidation reactions, there is a need to have a system wherein the ceric oxidant can be sufficiently stable under oxidizing conditions to be useful in indirect electrochemical processes, to be capable of undergoing repeated cycling between its cerous (Ce.sup.+3) and ceric (Ce.sup.+4) species in a high degree of efficiency under the reaction and electrolysis conditions, to be highly selective in forming the desired carbonyl group containing compounds, to be capable of exhibiting high reaction rates to make the process attractive on a commercial scale, to have high solubility to aid in the efficiency of the reaction and to eliminate the problems associated with slurries of cerium salts. It is readily seen that a means of achieving this combination of desired properties would aid in providing a process which would find a high degree of acceptance in electrochemical oxidation of aromatic and alkyl substituted aromatic compounds.